The present invention relates to the introgression of a quantitative trait loci (QTL) responsible for high sugar content from green-fruited tomato into cultivated tomato plants. More particularly, the present invention relates to tomato plant lines in which introgression of a green-fruited tomato QTL into a cultivated tomato genetic background resulted in a general cultivated tomato phenotype having fruits characterized by a higher sugar content and brix values.
Ever since the emergence of modern agriculture, cultivated plants have been manipulated in an effort to establish crops with agronomically important traits.
Such traits typically include, plant yield and quality, enhanced growth rates and adaptation to various growth conditions.
Presently a great deal of emphasis is placed on the generation of plants having desired traits via genetic engineering techniques. Over the past decade, advances have been made in developing methods of transferring genes to plant cells (see Potrykus et al., Plant Mol. Bio. Rep. 3:117–128 (1985)). For example, transfer and expression of single genes improving insect and herbicide resistance has reportedly been achieved in plants (Abel et al., Science 232:738–743 (1986); Shah et al., Science 233:478–481 (1986)). While there is excitement over advances in plant genetic engineering, the prospects for the general use of these techniques for plant improvement are tempered by the realization that very few genes corresponding to plant traits of interest have been identified or cloned. Furthermore, agronomically important traits such as, for example, plant yield, height, maturity, and fruit and grain characteristics, are all attractive targets for biotechnological manipulation techniques. However, these traits are often the result of the activity of several genes and as such, the use of direct gene transfer in manipulating these traits, is difficult due to problems in pinpointing and then cloning the individual loci which contribute predominantly to the expression of the trait.
As such, currently practiced approaches for producing plants having agronomically important traits generally rely on conventional plant breeding programs in which plants of a different genotype are genetically crossed in order to produce a hybrid with a recognizable agronomically important trait.
Such an agronomically important trait may be an easily recognizable morphological characteristic such as, for example, fruit size or color, or alternatively traits which are difficult or expensive to evaluate may be selected for by using an indirect selection criteria (Hallaver and Miranda, Quantitative Genetics in Corn Breeding Iowa State University Press 1981). One indirect selection criterion, for example, might be an easily recognized morphological characteristic of the plant which is either genetically linked to the desired trait or perhaps a component of the desired trait, e.g., the association between leaf size and seed size in beans.
Although conventional plant breeding has been used extensively for producing plants having agronomically important traits, such an approach is often difficult to apply to quantitative inherited traits which are determined by the activity of quantitative trait loci (QTL).
Thus, influencing heritability of quantitative inherited traits is difficult, because expression of a number of different gene products generally influences the phenotype. Quantitative traits are characterized by continuous rather than discrete distribution of phenotypic values. There is currently a poor understanding of how single genes influence the expression of complex traits and, in conventional plant breeding programs, selection for inheritance of quantitative traits is difficult due to the unrecognized genetic basis of the trait (Berger, Proceedings of the International Conference on Quantitative Genetics (Pollack et al., Eds., p. 191–204, Iowa St. Press 1977).
Complete linkage maps of DNA markers have facilitated mapping of genes affecting quantitative inherited traits (Paterson et al, Genetics 127: 181–197, 1991 and Tanksley Annu. Rev. Genet. 27:205–233, 1993). Such marker maps must be produced from a suitable mapping population which posses sufficient polymorphism for marker analysis and for quantitative traits. For a self pollinated crop such as for example, tomato, little variation between cultivated varieties is detectable by DNA markers. To overcome this problem, studies of quantitative loci were performed on wide crosses between species or races.
The cultivated tomato (Lycopersicon esculentum) is a self-pollinated vegetable crop which is widely cultivated. Tomato's well-endowed genetic resources include 877 monogenic mutations and more than 1000 accessions representing eight wild species. More than 1000 RFLP markers, spanning 1200 centimorgans (cM), have been positioned on the tomato linkage map, providing the basis for resolving quantitative traits into discrete Mendelian factors. The self-pollinated nature of the cultivated tomato enables the construction of populations that segregate for two alleles only at each locus, thereby simplifying analyses of the associations between markers and quantitative traits. Most quantitative trait loci (QTL)-mapping studies in tomato have been conducted on progenies of interspecific crosses, because within L. esculentum there is very low DNA-marker and phenotypic variation. Another reason for attempts to map QTL originating from exotic germplasm arise from the fact that the genome of the cultivated tomato, as is the case for many other crop plants, represents only a small fraction of the variation present in the gene pool of the species.
Various tomato plant lines in which introgressions of wild tomato were introduced into a cultivated tomato genetic background were generated in efforts to isolate quantitative trait loci.
One of the major objectives in tomato breeding is to increase the content of total soluble solids (TSS or brix; mainly sugars and acids) of the fruits in order to improve taste and processing qualities. It is known that the TSS content in fruits of wild Lycopersicon species can reach up to 15% (15 brix units) of the fresh weight, which is three times higher than cultivated varieties grown under similar conditions. To resolve the genetic basis for this variation, Eshed and Zamir (Genetics 141: 1147–1162, 1995), generated a set of 50 introgression lines from a cross between the green-fruited L. pennellii and the cultivated tomato, L. esculentum. Each of the lines contained a single RFLP defined L. pennellii chromosome segment, and together the lines provide complete coverage of the genome. Using this approach the study performed by Eshed and Zamir demonstrated mapping of 23 QTLs that regulate brix. Although the hybrid plants generated by this study were characterized by a high brix value, phenotypically such plants were further characterized by a small fruit mass, large foliage non-uniform ripening, large internodes and other undesirable characteristics which make such plants unsuitable candidates for commercial applications.
In further studies conducted by Eshed and Zamir (Genetics 143: 1807–1817, 1996) an attempt was made to further isolate the high brix value QTLs. This study uncovered that epistasis, which is a case in which one gene masks or interferes with the phenotypic expression of one or more genes at other loci, plays a key role in QTLs affecting fruit mass and total soluble solids, suggesting that some QTLs that affect these continuous traits in the same direction, interact in a less than additive manner.
Although these studies characterized several QTLs which are thought to be responsible for high brix value, introgression of the most prominent QTL into a cultivated tomato background introduced therein adverse genetic traits, such as high green percentage and long internodes, rendering the resulting tomato plants inapplicable for commercial purposes.
The present invention relates to the dissection of the introgression described by Eshed and Zamir in Genetics 143: 1807–1817, 1996, so at to obtain cultivated tomato plants having fruits characterized by increased brix values, yet are devoid of the adverse genetic traits.
The phenotypic and marker data for all lines have been previously published (Eshed and Zamir 1995), except for IL2-6-1, IL 9-2-5 and IL 12-1-1, which were derived from the fine mapping of QTL of their parental ILs (IL2-6, IL9-2 and IL 12-1; Eshed and Zamir, unpublished results). IL9-2 and IL12-1, with introgressed segments of 37 and 15 cM, respectively, were trimmed to generate IL9-2-5 and IL12-1-1 (9 and 4 cM, respectively). To unify data representation, the deviation of each ILHab from its expected value (interaction effect) is presented in Δ% from M82. For example, ILH1-1 increased B by 15% as compared to M82; ILH9-2-5 increased B by 22% as compared to M82. The expected effect for the hybrid between the two homozygous ILs (IL1-1 and IL9-2-5) is a 37% increase in B relative to M82. The observed B for the hybrid heterozygous for the two introgressions was only 26% higher than M82, indicating a significant interaction.